The invention relates to an implant and to a method of manufacturing an implant packed in a sterile manner.
Ultra-high molecular weight polyethylene (UHMWPE) is a material widespread in orthopaedic endoprosthetics which has proven itself as a sliding partner with respect to metallic and ceramic materials. A problem with these material pairings is the wear unavoidable in use and the particle-induced osteolysis triggered thereby which in the worst case can result in a loosening of the implant.
A suitable measure for the optimisation of the UHMWPE with the aim of reducing the arising wear is high cross-linking. This is achieved by electron radiation or gamma radiation and a heat treatment of the material, whereby the ultra-high molecular weight polyethylene is made wear resistant and given long term stability. The high cross-linking takes place by a cross-linking of the UHMWPE molecule chains and by a desaturisation of the free radicals.
The sterilisation method of gamma radiation usual in orthopaedic endoprosthetics cannot be used for highly cross-linked UHMWPE, because unwanted free radicals would again thereby arise. These promote an oxidisation effect and an embrittlement of the material associated with this. Highly cross-linked UHMWPE is therefore not sterilised with ionising radiation, but with gas, for example with ethylene oxide or gas plasma.
With implant components which consist exclusively of highly cross-linked polyethylene, satisfactory results are achieved by means of high cross-linking and heat treatment of the UHMWPE blank, subsequent mechanical processing, packaging and gas sterilisation.
However, implants can consist of a plurality of elements put together during the manufacturing process. With such composite implants, the problem of the effective sterilisation arises. A sterilisation with gamma rays would cover the whole implant and thus oxidise elements made of UHMWPE. A gas sterilisation, in contrast, only acts on all those surfaces of the implant which are accessible to the gas during the sterilisation. To also make surfaces covered by additional elements accessible to the sterilising gas, it is known in this connection to carry out the connection of implant elements with play. However, such play is problematic on the basis of clinical experience, since micro-movements of implant elements made possible in this manner are under suspicion of producing unwanted wear and of countering a permanent ingrowth of the bone.